If I hatch my Nightmare with another pet would its offspring cast unicorn too? Sorry I don't know much about hatching? I've just never bothered with it until now.
It might, but isn't for sure. You have about a 50/50 chance for the talent to pass to the new pet, and then about a 50/50 chance to unlock it, so it might take several tries to get it from hatching, but you can do it.